Coccyzus cuckoos are cuckoos are very slender birds with slender, slightly curved bills and long, strongly graduated tails; in most species the rectrices have a large pale spot at the tip. Dark-billed Cuckoo is mostly brown above, with a grayish crown, and a blackish band ("mask") across the side of the face. The underparts are buff. The underside of the tail is black, each rectrix with a large white spot at the tip. Dark-billed Cuckoo has a black bill, and a narrow yellow eyering.
Dark-billed Cuckoo is readily identifiable under most circumstances, from the ombination of black bill, buff underparts, and large white spots on the underside of the tail, but this species superficially similar to several other species of Coccyzus.
Gray-capped Cuckoo (Coccyzus lansbergi) has rufous brown underparts, a more distinct gray crown, and lacks the dusky "mask" across the face of Dark-billed Cuckoo.
Black-billed Cuckoo (Coccyzus erythropthalmus) has smaller tail spots, the undersurface of the tail is gray; and the face lacks a black mask; also, the underparts of Black-billed Cuckoo is paler (white or whitish), and the eyering is red, not yellow.
Mangrove Cuckoo (Coccyzus minor) is larger than Dark-billed Cuckoo, with an orange base to the mandible of the bill, and gray brown upperparts.
The following description is based on Payne (1997, 2005):
Adult: Sexes Long, slim shape, typical of Coccyzus. Crown and nape gray. Upperparts grayish brown, wings darker than back. Broad black stripe ("mask") across side of head, extending from the base of the bill through and past the eye. Underparts pale buff below, deepest buff on breast; sides of breast and flanks gray. Flight feathers show no rufous. Tail is long and graduated. Upper surface of rectrices brown; under surface of central pair rectrices brown with black tips, next pair blackish with narrow white tips, under surface of remaing rectrices black with broad white tips.
Juvenile: Similar to adult, but duller. Crown and nape brown. Wings sometimes rufous; coverts with indistinct buff tips. Rectrices narrow, pointed, and with indistinct gray tips.
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Iris: dark brown, black; narrow orbital ring of bare skin citron yellow, olive yellow, or gray
Tarsi and toes: lead gray, slate gray, olive, black.
Bare parts colors from Haverschmidt (1968), Belton (1984), and Payne (2005).
Total length: 25 cm (Hilty and Brown 1986), 27 cm (Hilty 2003), 27-28 cm (Ridgely and Greenfield (2001), 28.3 cm (Sick 1993)
Linear measurements (Payne 2005):
wing length, male (n = 9; Paraguay), mean 117.9 ± 3.2 mm (range 113-123 mm)
tail length, male (n = 9; Paraguay), mean 135.6 ± 7.5 mm (range 126-148 mm)
bill length, male (n = 9; Paraguay), mean 22.3 ± 0.8 mm (range 21.2-24 mm)
tarsus length, male (n = 9; Paraguay), mean 24.3 ± 0.9 mm (range 24.3-26.1 mm)
wing length, female (n = 11, Paraguay), mean 119.2 ± 2.6 mm (range 115-123 mm)
tail length, female (n = 11, Paraguay), mean 137.2 ± 5.0 mm (range 134-144 mm)
bill length, female (n = 11, Paraguay), mean 22.6 ± 1.5 mm (range 20.2-24.5 mm)
tarsus length, female (n = 11, Paraguay), mean 24.0 ± 0.6 mm (range 23.2-25.2 mm)
Mass: male, mean 45.6 g (range 42-54 g, n = 14; Payne 2005); female, mean 54.2 g (range 50-66.5 g, n = 7; Payne 2005)